Etiopathogeneamsis if RHD

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Here is a detailed, well-sourced account of the etiopathogenesis of Rheumatic Heart Disease (RHD).

Etiopathogenesis of Rheumatic Heart Disease (RHD)

RHD is the cardiac manifestation of Acute Rheumatic Fever (ARF), itself an immunologically mediated complication of Group A streptococcal (GAS) pharyngeal infection. It is fundamentally a disease of three interacting factors: the agent (GAS), the susceptible host, and an adverse environment.

1. The Agent - Group A Streptococcus (GAS)

ARF follows pharyngeal infection with GAS (Group A beta-hemolytic Streptococcus pyogenes), not skin infection in most populations (though GAS skin infections may contribute in Africa via scabies-related pyoderma).
The attack rate from untreated epidemic GAS pharyngitis is 1-3%, indicating that only genetically susceptible individuals develop ARF.
The pathogenic potential resides in specific streptococcal antigens, particularly the M protein (an alpha-helical coiled-coil cell surface protein). Certain M protein serotypes (rheumatogenic strains) are especially likely to trigger the cross-reactive immune response.
The group A carbohydrate (specifically N-acetyl-beta-D-glucosamine, GlcNAc) is another key antigen.

2. Molecular Mimicry - The Core Mechanism

The central pathogenetic mechanism is molecular mimicry: GAS antigens share structural/epitopic similarity with host cardiac proteins, causing the immune response to attack the heart.

GAS Antigen	Cross-reacts with Host Protein
M protein (especially M5, M6)	Cardiac myosin (light meromyosin, LMM)
M protein	Tropomyosin, vimentin
N-acetyl-glucosamine (GlcNAc)	Cardiac myosin, laminin (valve basement membrane)
M protein epitopes	Neuronal antigens (caudate nucleus) - causes chorea

The key epitope on M5/M6 protein is a five-amino-acid sequence Gln-Lys-Ser-Lys-Gln that cross-reacts with cardiac myosin. Up to 63.2% of intralesional T cell clones from RHD valve tissue react against cardiac light meromyosin peptides. - Firestein & Kelley's Textbook of Rheumatology

3. The Immune Response - Step by Step

Step 1: Innate immunity activation

GAS pharyngitis triggers neutrophils, macrophages, and dendritic cells to phagocytose bacteria and present antigens to T cells.

Step 2: Adaptive immune response

Humoral: Cross-reactive antibodies (anti-M protein, anti-GlcNAc) are generated. These antibodies bind to valve endothelial cells and basement membrane (cross-reacting with laminin), generating inflammatory signals.
Cellular: CD4+ T cells are activated. Intralesional T cells in valve tissue cross-react with streptococcal M5 peptides AND cardiac myosin, confirming dual specificity.

Step 3: The "Two-Hit" Hypothesis for valve damage

First hit: Anti-GlcNAc and anti-myosin antibodies bind valve endothelium → upregulate VCAM-1 (vascular cell adhesion molecule-1) on the endothelial surface.
Second hit: Upregulated VCAM-1 interacts with VLA-4 (very late activation antigen-4) on CD4+ T lymphocytes → T cells extravasate into valve tissue.

This results in a cytokine-rich, T cell-driven chronic valvulitis.

Step 4: Cytokine-mediated amplification

Four T-helper subsets (Th1, Th2, Th17, Tregs) participate:

TNF-alpha, IFN-gamma (Th1) - drive pro-inflammatory macrophage activation
IL-4, IL-10 (Th2) - modulate humoral response
IL-17, IL-23 (Th17) - promote neutrophil recruitment and tissue damage

Key chemokines recruited to valve tissue include CCL1/I-309 and CXCL9/Mig, which mediate both CD4+ and CD8+ T cell infiltration. Matrix metalloproteinase-25 (MMP-25) is implicated in extracellular matrix degradation in the valve.

4. The Host - Genetic Susceptibility

Only ~3-6% of individuals exposed to rheumatogenic GAS infection develop RF, suggesting genetic susceptibility is essential.
Heritability of RF is ~60%.
Individuals with a family history of RF have ~5x higher risk; children raised separately from parents with RHD still have a relative risk of 2.93.
Genetic associations found in:
- HLA class II alleles (controlling adaptive immune response)
- Innate immunity genes: ficolin-2, mannose-binding lectin-2, TLR2
- Cytokine genes: TNF-alpha, TGF-beta, IL-1RA, IL-10
- B-cell alloantigens (D8/17 antigen historically associated)
Women are approximately 1.8x more susceptible to developing RHD (also influenced by pregnancy exposure and child-rearing proximity to GAS).

5. The Environment

RF is a disease of poverty:

Associated with overcrowding, poor housing, malnutrition, low income, limited healthcare access.
The incidence fell in industrialized countries before the antibiotic era, driven by improved socioeconomic conditions.
In endemic regions (Sub-Saharan Africa, South Asia, Pacific Islands, indigenous Australian/New Zealand communities), incidence among 5-14 year-olds can reach 162-228/100,000/year.

6. Pathologic Features (Acute ARF → Chronic RHD)

Acute Phase - ARF causes Pancarditis

The pathology affects all three layers of the heart:

Layer	Acute Finding
Pericardium	Fibrinous exudate (usually resolves without sequelae)
Myocardium	Aschoff bodies (pathognomonic)
Endocardium/Valves	Fibrinoid necrosis + 1-2mm fibrin vegetations (verrucae) along valve closure lines

The Aschoff Body (Pathognomonic Lesion)

Aschoff body - photomicrograph showing Anitschkow cells (caterpillar cells) with central fibrinoid necrosis surrounded by mononuclear infiltrate

Aschoff body: composed of T lymphocytes, plasma cells, and plump activated macrophages called Anitschkow (caterpillar) cells - these have abundant cytoplasm and a central wavy chromatin ribbon resembling a caterpillar. Surrounded by a zone of fibrinoid necrosis. - Braunwald's Heart Disease, p. 856

Chronic Phase - Scarring and Valvular Distortion

Repeated episodes of ARF → cumulative valve damage → chronic RHD:

RHD gross and microscopic pathology: (A) verrucous vegetations on mitral valve; (B) Anitschkow cells; (C-D) mitral stenosis with commissural fusion and leaflet thickening; (E) rheumatic aortic stenosis

Fig 9.19 (Robbins & Kumar Basic Pathology): Acute rheumatic mitral valvulitis (A), Anitschkow cells (B), classic mitral stenosis with commissural fusion and chordal thickening (C-D), rheumatic aortic stenosis (E).

Key chronic changes:

Aschoff bodies replaced by fibrous scar
Valve leaflets thickened, retracted, fibrotic
Commissural fusion ("fishmouth" / "buttonhole" stenoses)
Thickening and fusion of chordae tendineae
Neovascularization within leaflets
Progressive calcification

7. Valve Distribution in Chronic RHD

Valve Involved	Frequency
Mitral alone	70%
Mitral + Aortic	~25%
Tricuspid (less severely)	Less common
Pulmonic	Almost never

Mitral stenosis is essentially the only acquired cause of isolated mitral stenosis. The left atrium dilates under pressure, leading to atrial fibrillation, mural thrombus formation, and eventually pulmonary hypertension and right heart failure. - Robbins & Kumar Basic Pathology, p. 366-367

Summary Diagram of Pathogenesis

GAS pharyngitis (rheumatogenic M-type)
         ↓
   Immune activation (innate + adaptive)
         ↓
  Cross-reactive antibodies & T cells (Molecular Mimicry)
     - Anti-M protein ←→ cardiac myosin, tropomyosin
     - Anti-GlcNAc ←→ laminin (valve basement membrane)
         ↓
  "Two-hit": antibody → VCAM-1 upregulation → T cell infiltration
         ↓
   Acute valvulitis (verrucae, Aschoff bodies)
         ↓
  Repeated ARF episodes (untreated strep pharyngitis)
         ↓
   Cumulative valve scarring → fibrosis → stenosis/regurgitation
         ↓
         CHRONIC RHD

Sources: Braunwald's Heart Disease (p. 855-856), Robbins & Kumar Basic Pathology (p. 366-367), Firestein & Kelley's Textbook of Rheumatology (pp. 2578-2579), Harrison's Principles of Internal Medicine 22E.

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